As copying and scanning of color documents becomes more prevalent, there has arisen a need for a solid-state electro-optical device suitable for sensing images, such as a silicon chip having an array of photosensors. For a photosensor to be sensitive to a specific primary color, a translucent filter layer, such as a polyimide layer that has been dyed or pigmented to the specific primary color, may be applied on the surface of the chip. If a single photosensitive chip is intended to have multiple linear arrays of photosensors, each linear array being sensitive to one particular primary color, particular polyimide filter layers are applied to specific linear arrays, thereby creating a full-color photosensitive chip.
FIG. 1 is a plan view of two photosensitive chips 10 fashioned from a single wafer 11. The chips 10 are of a general design found, for example, in a full-color photosensor scanner of the prior art. The chip 10 includes a number of bonding pads 12, and one or more linear arrays of photosensors 14. A typical design of a full-page-width scanner will include a plurality of chips 10, each chip being approximately one-half to one inch in length, the chips 10 being butted end-to-end to form a collinear array of photosensors 14, which extends across a page image being scanned.
Each chip 10 is a silicon-based integrated circuit chip having on a surface three independently functioning linear arrays of photosensors 14. The photosensors 14 are disposed in three parallel rows that extend across a main dimension of the chip 10, these individual rows being shown as 16a, 16b, and 16c. Each individual row of photosensors on the chip 10 can be made sensitive to a particular color by applying to the particular rows 16a, 16b, and 16c a spectrally translucent filter layer that covers only the photosensors in a particular row. For example, the three rows of photosensors can be filtered with a different primary color, such as red, green, and blue. Generally, each individual photosensor 14 is adapted to output a charge or voltage signal indicative of the intensity of light of a certain type impinging thereon. Various structures, such as transfer circuits, or charge-coupled devices, are known in the art for processing signal output by the various photosensors 14.
One method of constructing a full-color photosensitive chip 10 according to the prior art is to first construct a wafer 11 having a relatively large number, such as one hundred or more, semiconductor structures, each structure corresponding to one chip 10. Two such chips are shown in FIG. 1. For full-color chips, the wafer 11 is coated with multiple layers of translucent filter material by spin coating. A filter liquid, corresponding to a filter of a particular color, is poured near the center of the wafer 11, and then the wafer 11 is spun about an axis 17 to spread the liquid. The filter material may then be etched away as needed, to yield the three primary-color-filtered linear arrays of photosensors 14, as known to those of ordinary skill in the art. Only after the filter layers are applied as desired is the wafer “diced,” or sawed into individual chips.
In the foregoing method of fabricating a full-color photosensitive chip 10, a problem may arise when applying successive translucent filter layers. In particular, the process of applying a filter coat to the chip may cause the coat to be thicker on some photosensors than on others. Different thicknesses of the filter coat result in different intensities of light passing through the filter material to a particular photosensor. Such variations may result in diminished reproduction quality. For photosensors of a particular type on a single chip, it is desirable that the filter coat be of uniform thickness. In addition, when applying a filter coat, it is desirable to leave a smooth surface on the chip on which to apply the next filter coat. If the surface is not smooth, color reproduction quality can suffer.